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In October 1996, Palmerston North detective Brent Garner was found beside his scorched home, bound and gagged.

A crude pentagram had been carved into his back. He had survived a visit from "The Executioner", a Satanic maniac so bizarre, detectives on the hunt for him thought the only place he could exist was in the movies.

And now he does. Venus and Mars revisits the dual police investigations in the hunt for Garner's attacker.

Venus was the official investigation in which detectives believed The Executioner was real. Several weeks later a covert operation - Mars - was launched in which Garner was treated as a suspect. It was led by Inspector Grant Nicholls.

The police suspicions were right - the 32-year-old had faked the crimes so he could claim insurance money to build a new life without his wife and two young daughters.

Inside of Brent Garner's burned-out house.

Nicholls, now a Deputy Commissioner, said doubts about Garner's story prompted the second investigation but officers still had to take the attack claims seriously.

"You can imagine the scenario: there's effectively a homicidal Satanic-worshipping maniac hell-bent on murdering a member of police wandering around the country for months sending threatening letters," said Nicholls.

"He manages to circumvent security, burn down [Garner's] house and all but leave him for dead and escape jurisdiction. That had to be solved. At any cost."

It cost police $350,000. They completed 3,200 tasks, spoke to 1,959 people and took more than 1,000 photographs. But the toll it left on the people involved would last longer than the 37-day investigation.

The first letters arrived out of the blue on August 25, 1996. One landed on Garner's desk, the other given to the local newspaper. They threatened Garner's life, cited Satanic verse and were signed "The Executioner".

The letters were out of place in mid-90s Palmerston North. Satanists? Not there. And Garner worked on the fraud squad. He didn't deal with hardened thugs or have much to do with the underworld.

Identikit of Brent Garner's attacker.

"You have to ask yourself why, but you take it on face value and there's no good reason to disbelieve the intent," said Nicholls. "You can't discount the possibility that some disaffected individual got up one morning thinking this was a way of seeking retribution against the police."

On September 25, a third letter arrived, which upped the threats to such an extent that Garner's wife and daughters were relocated.

At 4am on October 17, Garner was found beside his house bound with cables and with a pentagram carved into his back. A fourth letter was lodged in his gagged mouth. His house had been burned. He said he'd escaped The Executioner.

The then-detective asked himself what kind of person could defeat the heavy security in the house. "Where does such an individual exist other than in the movies?"

Nicholls was at a crossroads. He was the second in charge of Operation Venus and the lead detective in Operation Mars — an operation so covert 10 officers working on the original investigation were assigned dual tasks.

They examined every piece of evidence in two contexts — one as if Garner was the suspect, and then as if The Executioner was real. They took over one floor of the old Palmerston North library building.

It was also the first time New Zealand Police used the internet to scour for information. Nicholls had to put in a written request to upload an identikit of the suspect on a website.

Five weeks later, when Garner was finally brought in for questioning, he knew he had been found out.

The wounds on Brent Garner's back.

Said Nicholls: "I felt very let down. Some [officers] were surprised, some I suspect weren't. The reactions were mixed."

When news filtered through the station that Garner had been taken in for questioning, his friends set off the fire alarms, intent on stalling the process.

But Garner was sitting in the old library where he was questioned for five hours. He was calm, didn't falter and phoned in a confession the next day.

He pleaded guilty to forgery, arson, false pretences, wasting police time and making a false complaint. He was sentenced to five years' jail and was released in August 1998. In 2003, his body was found in his car at an isolated Bay of Plenty Beach.

There are still "bits and pieces" about the case that eat away at Nicholls because Garner was "a decent fellow".

"I still have difficulty around what I call the slow-burning fuse. This was planned over a long time and he was still functioning normally in the office. It must have been incredibly difficult for him — setting it up, coming to work and looking his colleagues in the eye."

To go to such extremes to create a new life is unfathomable. Garner, who was having an affair with a colleague, wanted to liquidate his assets, separate from his wife and change the direction of his life.

"There are easier ways to do it. You don't have to go to these extremes."

Family's $2.6m 'slap in the face'

His mother, Lydia Garner, told the Herald on Sunday the family did not support TV One's drama, Venus and Mars. She said it was a "slap in the face" that $2.6 million in NZ On Air funding had been provided for the show on her son's offending.

"It is very unsettling. We're a loving family. We stuck together, we looked after one another, we've been through lots. Why bring it up again?" she said. "It is disturbing when we're trying to get on with our lives. We have succeeded and now they're trying to squash us down again."

Lydia Garner said her son's two daughters were now grown.

His wife at the time, Sandra, has remarried. Despite the twisted events of almost 19 years ago, Garner's parents remember him as their "very loving boy".

"He was a loving and caring person. I don't know why it happened but sometimes with their job they break," she said.

During filming in July last year, producer Philly de Lacey said there were valid reasons for telling the story.

"For them and for Sandra and their children, it's a part of their life that they're never going to forget," she said. "It's a part of their life that's never going to go away... unfortunately, their family member has become a part of a story that's quite historically extraordinary in New Zealand and that becomes a bit of the legacy."

Garner is played by Australian actor Ande Cunningham who spent "a lot of time in the darkest places of Brent Garner's mind" and reimagining his future.

"We spent a lot of time talking about and solving his motivations. He always put the best version of himself forward. He didn't realise what the aftermath was going to be," said Cunningham.

"I've spent a bit of time imagining what that new life would look like because it's an important part of his motivation. You can't ever get a full understanding of what that life looked like to him."

Venus and Mars also features Joel Tobeck, Craig Hall and Sara Wiseman.